Piston construction for high pressure apparatus

ABSTRACT

THE INVENTION PERTAINS TO A PISTON STRUCTURE FOR HIGH PRESSURE WORK IN WHICH A SMALLER END OF THE PISTON IS SUBJECTED TO PRESSURE AND WHEREIN THE PISTON IS DIVIDED INTO A FIRST SMALLER TAPERED END PART WHICH IS INTRODUCED INTO A CHAMBER SMALL END FOREMOST TO EXERT HIGH PRESSURE ON A SAMPLE IN THE CHAMBER, AND INTO A SECOND LARGER PART SEPARATE FROM THE SMALLER END PART AND WHICH LARGER PART DELIVERS THE THRUST FROM A PRESS TO THE LARGER END OF THE SMALLER END PART SO THAT IF PISTON BREAKAGE OCCURS IT WILL BE CONFINED TO THE SMALLER END PART.

O 19, 1 7 B. J. NEMETH ETAL 3,613,164

PISTON CONSTRUCTION FOR HIGH PRESSURE APPARATUS Filed Aug. 7, 1968 m 38Tl IIIIIH 36 FIG-2 IN VUN'R m5 8. J. NEMETH' B. c. BOECKELER MAWQMUnited States Patent 01' fice 3,613,164 Patented Oct. 19, 1971 3,613,164PISTON CONSTRUCTION FOR HIGH PRESSURE APPARATUS Bela J. Nemeth andBenjamin Clark Boeckeler, Greensburg, Pa., assignors to Kennametal Inc.,Latrobe, Pa. Filed Aug. 7, 1968, Ser. No. 750,809 Int; Cl. B30b 11/32US. Cl. 1816.5 Claims ABSTRACT OF THE DISCLOSURE The invention pertainsto a piston structure for high pressure work in which a smaller end ofthe piston is subjected to pressure and wherein the piston is dividedinto a first smaller tapered end part which is introduced into a chambersmall end foremost to exert high pressure on a sample in the chamber,and into a second larger part separate from the smaller end part andwhich larger part delivers the thrust from a press to the larger end ofthe smaller end part so that if piston breakage occurs it will beconfined to the smaller end part.

The present invention relates to high pressure apparatus of a generallyknown type in which two tapered pistons are introduced into a cavitysmall end foremost from opposite ends of the cavity to compact a Worksample therein and to develop extremely high pressures on the worksample.

The particular high pressure apparatus to which the piston of thepresent invention pertains is usually known as the belt or girdledesign. As mentioned, such devices, in general, are -well known in thehigh pressure field. Typical devices of this nature are shown, forexample, in US. Pats. 2,941,247 and 2,941,248.

Heretofore, however, it has been not uncommon for the pistons of suchdevices to fracture due to the high stresses developed therein inexerting the extremely high pressures at which samples are sometimesworked in the high pressure apparatus. Because of the high pressures towhich the pistons are subjected, they are necessarily made of a hardhigh compressive strength material such as cemented carbides of whichcemented tungsten carbide is an example. The pressures encounteredexceed the yield strength of the best steels and, therefore, materialssuch as cemented carbides are resorted to in order to make the apparatusworkable.

While cemented carbides, in particular, tungsten carbide, are adequatefor sustaining the pressures imposed thereon with respect to resistingdeformation, it is nevertheless the case that the pistons will sometimesfracture. With devices of the known type, the piston is a rather largemember having a small tapered end part at one end that is introducedinto the cavity or chamber of the high pressure apparatus and asubstantially larger cylindrical opposite end part at the other endintegral with the small end part and which is subjected to pressure asby the platen or bed of an hydraulic press or other pressure exertingmechanism. The larger cylindrical part is usually slightly tapered andis held in a heavy steel ring by being press fitted or shrink fittedtherein by known procedures. With a piston constructed in the mannerreferred to above, breakage of any part of the piston involvesdiscarding of the entire piston. Such pistons are naturally quiteexpensive and are difficult to make and breakage of a piston is,therefore, a serious matter.

Furthermore, when the pistons are made as large members including both asmall end and a large end, a rather large body of the cemented carbidemust be worked and the possibility of pits and flaws and voids withinthe body of the carbide is greater than if a smaller body of carbide isworked. Since the carbide in the piston is being worked in a rangesomewhere near the limits of the strength thereof, when the piston is inservice it follows that any flaws whatsoever in the body of the carbidehave a strong tendency to induce breakage of the piston and therebymaterially shorten its working life and increase the cost of operatingthe high pressure apparatus. Furthermore, since the larger ends of thepistons are mounted in heavy steel support rings, the replacement of apiston involves considerable time and labor. This is especially truebecause the mounting of the piston in the steel ring must be done withgreat precision.

According to the present invention, the foregoing problems that haveexisted in connection with high pressure apparatus for a long time aresolved by the relatively simple expedient of making the small taperedend of the piston which enters the chamber of the high pressureapparatus separate from the larger end thereof which is subjected topressure by the pressure developing mechanism, such as a hydraulicpress. For the sake of clarity and convenience, in the followingdescription and claims, the smaller end of the piston structure isreferred to as a piston tip while the larger end is referred to as apressure block.

With the piston so divided, breakage is substantially confined to thesmaller separate end, or piston tip portion of the piston structure sothat, in the event of breakage, less loss is involved and less loss oftime occurs.

Unexpectedly, it has been found that misalignment of the piston tip withthe working chamber, and the development of regions of extreme stress inthe piston tip at the interface of the piston tip and pressure block,represents no serious problem and can easily be completely compensatedby the provision of a shim or washer of suitable hardness between thepiston tip and the pressure block.

The, forming of the small end of the piston structure as a separatemember insures a higher quality for this member because a smaller volumeof cemented carbides is being worked. Furthermore, the smaller end ofthe piston, when it is made as a separate member, can be compounded soas to have the very best qualities for its intended purpose.

Further, the pressure block can also be compounded to have the mostdesirable qualities. For example, while the piston tip must havehardness and high compressive strength, it must not be so brittle thatit will easily fracture. The pressure block, on the other hand, works atless stress than the smaller part of the piston tip and can be made morebrittle and thus more resistant to deformation.

The exact nature of the present invention will become more apparent uponreference to the following detailed specification, taken together withthe accompanying drawings in which:

FIG. 1 is a somewhat diagrammatic sectional view showing a typical highpressure apparatus and including therein piston structures according tothe present invention;

FIG. 2 is an elevational view of a piston and pressure block thereforaccording to the present invention drawn at enlarged scale; and

FIG. 3 is a fragmentary sectional view drawn at still greater scaleshowing some of the pertinent dimensional characteristics of the pistonand the pressure block therefor.

Referring to the drawings somewhat more in detail, the apparatus showntherein comprises a belt or girdle 10 press fitted or shrink fitted intoan outer supporting steel ring 12. Belt 10 forms a central chamber intowhich the upper piston 14 and lower piston 16 extend small end foremost.Within the chamber is a sample 18 which is to be subjected to highpressure. The pressure considered in connection with apparatus of thisnature may rise to nearly 100,000 atmospheres, although pressures below50,000 atmospheres are more common. The sample is usually containedwithin an electrically conductive tube 20 and an electrically conductivedisk 22 is positioned at each end of tube 20 and in electrical contacttherewith. Each disk 22 is engaged by a ring of electrically conductivematerial, indicated at 24, by means of which the disk is maintained inelectrical contact with its respective pistons 14, 16.

A sleeve-like gasket 26 of electrical insulating materia surrounds tube20, and disks 28 of electrical insulating material are disposed insiderings 24.

Between belt and sleeve 26 are gasket members 30 which seal the highpressure chamber. Gaskets 30 are advantageously made of two parts, anouter part 30a adjacent the ring 10 and extending into surroundingrelationship with sleeve 26, and an inner part 30b which is engaged bythe tip of the pertaining piston and which also surrounds the pertainingring 24. A conical metal element 32 is disposed between the parts 30aand 30b of gasket 30.

The sleeve 26 and disks 28 and the gaskets 30 are made of materialhaving the properties of thermal insulation and which are electricallynon-conductive and which are somewhat deformable at high pressureswithout, however, losing shear strength. The inner ends of the gasketsare at the high internal pressure of the cell while the outer ends areat atmospheric pressure so that maintenance of shear strength is of theutmost importance to prevent the gaskets from being extruded frombetween the piston tips and the belt. A number of materials are suitablefor this purpose and among such materials are pyrophyllite, catlinite,talc, and other stones and ceramic materials. With respect at least tothe gaskets 30', it is important that they confine the extremely highpressures developed within the working chamber of the apparatus while,at the same time, they permit advancement of the piston tips 14, 16,into the chamber. These materials must have the quality of flowing underpressure without, however, being flowable to the degree that pressurewill be lost within the working chamber.

Each piston tip, as will be seen in FIG. 1, tapers outwardly from itsworking end to a larger base region, and the base region is engaged by apressure block 34. Each pressure block 34 is substantially larger indiameter than its pertaining piston tip and is also preferably formed ofa cemented carbide. Each block 34 is preferably tapered on its peripheryand is press fitted or shrink fitted into an annular supporting steelring 36. The upper pressure block may be engaged by a platen 38 of apress while the lower pressure block rests on bed member 400 of thepress. In this manner, the necessary pressure can be exerted on thepressure blocks to drive the piston tips into the working chamber of thehigh pressure apparatus.

Turning now to FIGS. 2 and 3, it will be noted therein that between thebase end of each piston tip and the pressure block pertaining thereto isa metal disk 42 of a relatively hard material, hard steel, for example,but which is nevertheless somewhat softer than the carbide material ofthe pressure block and the piston tip. The provision of disk 42 permitsirregularities in the base end surface of the piston tip and in thesurface of the pressure block adjacent thereto to be compensated, andalso permits slight misalignments of the piston tip with the pressureblock to be compensated and, furthermore, protects the base member fromfragments of the piston tip in the event of failure thereof by absorbingor deflecting broken piston tip parts to prevent them from being driveninto or impacting against the base member.

The piston tip shown in FIGS. 2 and 3, which is piston tip 16 of FIG. 1;will be seen to be a substantially frustoconical member having a smallerwork end 44 which is introduced into the chamber of the high pressureapparatus, and a larger working end 46 to which pressure is ap- '4 1plied by the pertaining pressure block 34. The peripheral wall of thepiston is in the form of a smooth straight cone and the ends are alsostraight, being in the form of flat planes. At the junctures of the endfaces with the sidewall, radii are formed so as to eliminate regions ofextremely high stress in the piston.

As will be seen in FIG. 3 the curvature where the base surface 46 of thepiston joins the side wall thereof is indicated R1 and this radius maybe up to about 10% of the maximum diameter of the piston. The same, or asmaller, radius can advantageously be used where working end face 44joins the sidewall of the piston.

FIG. 3 will also show that the end face 46 of the piston tip may have aslightly tapering peripheral portion 48 and that, likewise, pressureblock 34 also has a tapering peripheral portion 50. The radius R2 of thecentral planar portion of base 46 of the piston tip is smaller thanradius R3 of the central planar portion of the end of block 34 adjacentto the piston so that the piston tip end will always be fully supportedon a flat surface even if there is some slight lateral shifting of thepiston tip relative to the pressure block.

The angle A pertaining to the sidewall of the piston tip can vary fromabout 10 to 45 degrees and it has been found that an angle of 20 degreesproduces highly satisfactory results.

The angle B pertaining to the tapered peripheral portion of the largerend of the piston tip may also vary considerably, say, from 0 to about15 degrees with satisfactory results.

The angle C pertaining to the tapered peripheral portion of pressureblock 34 can vary from 0 to 50 degrees and is not critical. Angles onthe order of 45 degrees have proved quite satisfactory for this purpose.The 45 degree taper at the working end of the pressure block permitseasy grinding of the flat end of the block to restore it to a planarcondition after the piston tip has been used long enough to detract fromthe flatness of the working end thereof.

The taper of the piston tip is provided so that the stress per unit areaat the working end 44 of the piston tip, and which approaches themaximum permissible stress of the material in the piston tip issubstantially reduced at the base end 46 thereof. The reduction in unitworking stress is such that there is little chance that the piston tipwill fracture at its larger end; such fractures ordinarily commencingnear the smaller working end of the piston. Further, the rounded cornerat the larger end of the piston tip prevents the extreme outer edge ofthe larger end of the piston tip from being stressed by the pressureblock to the point of fracturing the piston tip. The size of pressureblock 34 is selected so that the unit working pressure at the sidethereof to which pressure is applied by the member 38 or 40 is below theyield strength of common steel castings.

It will also be noted that the arrangement is such that an electricalcurrent can be passed through the sample in the working chamber in orderto develop high temperatures therein if so desired, although it will beunderstood that the invention is equally useful in both cold and hotprocesses.

From the foregoing it will be appreciated that the separate piston tips,according to the present invention, are easler to manufacture than theintegral piston tips and pressure blocks according to the prior art.

The smaller piston tip made in accordance with the present inventionwill weigh only about 12% as much as the pressure block and only about11% as much as if the piston tip and pressure block were made in asingle piece. Inasmuch as the piston tip is in the form of a smallerpiece, the quality thereof is better because it will containproportionately fewer surface and internal flaws. which could lead topremature failure.

Furthermore, it is less expensive and less time consuming to grind thesmall piston tip than to grind an entire piston tip and pressure blockcombination.

Also, since failure substantially always occurs in the piston tip, it isthis particular item which has to be replaced. The pressure blocks areordinarily fitted within a heavy retaining steel ring and it will beobvious that it is a much simpler, and a much cheaper, matter to replacea broken piston tip than it is to replace a larger unit consisting of apiston tip integral with a pressure block and the whole fixed by pressfitting or shrinking into a heavy steel ring.

It will furthermore be evident that the design of the present inventiondoes not in any way interfere with the passage of current through theapparatus to develop high temperatures within the sample in the cavityof the high pressure apparatus if it is desired to do so.

A further benefit of making the piston tip separate from the pressureblock is that one and the same pressure block can accommodate pistontips of different size and shape thereby substantially reducing toolinventory. Also, it has been found that piston tip life can be enhancedby periodic reheating thereof which seems to relieve stresses set up bythe deformation which the piston tip undergoes in use. Such reheating isnecessary only for the piston tip and can be done quite easily when thepiston tip is separate from the pressure block, as taught by the presentinvention.

Modification of the present invention falling within the scope of theappended claims will occur to those skilled in the art.

We claim:

1. In a high pressure apparatus; a ring of hard high strength materialforming a high pressure chamber circular in cross section and havingoutwardly tapering openings at the opposite ends, axially symmetricaltapering frusto conical piston tips of hard high strength cemented metalcarbide material, each said piston tip having flat parallel end facesand rounded corners at the juncture of the side wall thereof with saidend faces and receivable small end foremost in a respective saidopening, tapering frusto conical gasket means having inside surfacesengaging said piston tips and outside surfaces engaging the confiningwalls of the said openings and deformable under pressure, axiallysymmetrical pressure blocks of a hard high strength cemented metalcarbide material separate from said piston tips and coaxial therewithand engaging the large ends of said piston tips for exerting thrustthereon, each said pressure block being substantially larger in crosssectional area than its pertaining piston tip so that the unit pressuredeveloped in the block when pressed against the piston tip will besubstantially lower than the unit pressure developed in the large end ofthe pertaining piston tip, each said block having a flat circularcentral region larger in diameter than the large end of the pertainingpiston tip on the side facing the pertaining piston tip, each saidpiston tip being free of lateral support between the respective saidgasket means and the pressure block pertaining thereto, and a relativelythin metal disk larger in diameter than the large end of the piston tipinterposed between the large end of each piston tip and the said centralregion of the pertaining pressure block, said metal disk being of suchhardness as to prevent extrusion thereof laterally when the respectivepressure block is pressed against the respective piston tip but softerthan the material of said pressure block and piston tip so as to bedeformable to distribute stresses uniformly over the interengagingsurfaces of said pressure block and piston tip.

2. An apparatus according to claim 1 in which said metal carbide istungsten carbide.

3. An apparatus according to claim 1 in which each said pressure blockalso includes an annular region tapering outwardly and upwardly from theperiphery of said fiat central region to the outer periphery of theblock.

4. An apparatus according to claim 1 in which each piston tip, at theend thereof adjacent its pertaining pressure block, has an annularperipheral region which tapers away from the pressure block toward theadjacent rounded corner of the piston tip thereby providing the pistontip with a circular central planar area opposed to the said fiat centralregion of the pertaining pressure block.

5. An apparatus according to claim 4 in which the said annularperipheral region of the piston tip at the end thereof adjacent thepressure block which tapers from the central planar area of the pistonto the adjacent rounded corner is disposed at an angle of up to about 15degrees from the plane of the said central planar area.

References Cited UNITED STATES PATENTS J. HOWARD FLINT,

US. Cl. X.R.

1816 R, high pressure dig.

JR., Primary Examiner

